To achieve low emissions of undesirable combustion products for gas turbine engines, lean pre-mixed combustion systems are used. These systems have a pre-mixing zone for creating a controlled fuel/air mix, a reaction zone for combusting the fuel/air mix and a dilution zone for adding air to the combustion products.
These types of combustion systems are sensitive to the fuel/air ratio produced at the pre-mixing zone. There is an optimum value of flame temperature for low NOx emissions. If the percentage of fuel increases beyond the optimum, then NOx emissions increase as the flame temperature increases. If the percentage of fuel decreases below the optimum then CO emissions increase and the combustor may go out because the flame temperature has fallen too low.
A gas turbine requires varying amounts of fuel depending upon the required output from the engine. It is important that as the fuel required by the engine varies, the temperature in the reaction zone remains substantially constant at or near the optimum value. This temperature is controlled by fuel composition, the air:fuel ratio; and the degree of pre-heating of air and fuel prior to combustion. Therefore as more fuel is injected, more air is required in the pre-mixing zone and as less fuel is injected, less air is required in the pre-mixing zone.
There are a number of mechanisms in the prior art for varying the mass flow of air to be mixed with fuel prior to combustion. U.S. Pat. No. 4,255,927 and EP0547808 disclose a combustion system in which the air and fuel are mixed within a combustion chamber, without pre-mixing. An air flow from a compressor to the combustor is divided between the reaction zone of the combustor and the dilution zone of the combustor. An external valve mechanism is used to control the relative proportions of air flowing to the reaction zone and the dilution zone.
“Variable Geometry Fuel Injector for Low Emissions Gas Turbines”, by K. Smith et al, Solar Turbines Inc., Aeroengine Society of Mechanical Engineers (ASME) 99-GT-269, discloses a mechanism for varying the air flow to a pre-mixing zone of a fuel injector. This document discloses a lean pre-mixed combustion system in which a variable geometry injector uses a movable air metering plug at an upstream end of the injector to variably control the amount of air entering the pre-mixing zone. A nearly constant peak flame temperature during operation of the engine is maintained by moving the air metering plug. A problem with this type of system is that a change in the fuel injector geometry may result in a change in the total combustor area for fluid input with a consequent change in combustor pressure drop.
U.S. Pat. Nos. 3,927,520 and 5,309,710 disclose a variable geometry combustion systems that vary the amounts of air provided to the pre-mixing zone and the dilution zone without varying the combustor area for fluid input.
U.S. Pat. No. 3,927,520 discloses the control of air flow into the dilution zone, by using a first perforated sleeve movable to cover the dilution air ports, and the control of air flow into the pre-chamber, for pre-mixing with fuel, by using a second perforated sleeve movable to cover the air entrance ports. The sleeves operate so that the exposed area of the second entrance ports in the pre-chamber varies in the reverse sense to the exposed area of the dilution air ports.
U.S. Pat. No. 5,309,710 discloses a combustion system that maintains a nearly constant peak flame temperature during operation of the engine cycle by using variable geometry air flow control. A plurality of poppet valves are located adjacent the mixing zone of the combustor chamber. Each poppet valve is in one of two configurations—either an open position in which air is directed into the reaction zone or in a closed position in which the air is directed to the dilution zone. A poppet valve therefore directs air to either the mixing zone or the dilution zone but not both. The system is designed so that the open combustor area is the same whether or not a port is open or closed. Thus the open area of the combustor is kept constant.
Variable injector geometries are known from other applications. JP 7-318059 & JP 7-280266 disclose a combustion system with a moveable centre body injector. Fuel is injected from the tip of the centre body. The combustion system has a first pre-mixing configuration in which the centre body is drawn into a pre-mixing area where injected fuel and air mix. The combustion system has a second configuration in which the centre body protrudes into the reaction zone and fuel is injected directly into the reaction zone without pre-mixing with air.
A problem with existing variable geometry pre-mixing injectors is that they are susceptible to flashback, in which the combustion flame jumps inside the injector. This can cause damage if the flame reaches the fuel inlet.
It would be desirable to provide a pre-mixed variable geometry combustion system that operates at or near a predetermined fuel/air mix without producing significant changes in combustor pressure drop, while reducing the likelihood of damage arising from flashback.